Paramagnetic NMR Relaxation in Polymeric Matrixes: Sensitivity Enhancement and Selective Suppression of Embedded Species (1H and 13C PSR Filter)

A study of the practical applications of the addition of paramagnetic spin relaxation (PSR) ions to a variety of polymers (PLL, PAA, PGA, PVP, and polysaccharides such as hyaluronic acid, chitosan, mannan, and dextran) in solution (D₂O and DMSO-d₆) is described. Use of Gd^III, Cu^II, and Mn^II allows a reduction of up to 500% in the ¹H longitudinal relaxation times (T₁), and so in the time necessary for recording quantitative NMR spectra (sensitivity enhancement) neither an increase of the spectral line width nor chemical shift changes resulted from addition of any of the PSR agents tested. Selective suppression of the ¹H and¹³C NMR signals of certain components (low MW molecules and polymers) in the spectrum of a mixture was attained thanks to their different sensitivity [transverse relaxation times (T₂)] to Gd^III (PSR filter). Illustration of this strategy with block copolymers (PGA-g-PEG) and mixtures of polymers and low MW molecules (i.e., lactose−hyaluronic acid, dextran−PAA, PVP−glutamic acid) in 1D and 2D NMR experiments (COSY and HMQC) is presented. In those mixtures where PSR and CPMG filters alone failed in the suppression of certain components (i.e., PVP−mannan−hyaluronic acid) due to their similarity of ¹H T₂ values and sensitivities to Gd^III, use of the PSR filter in combination with CPMG sequences (PSR−CPMG filter) successfully resulted in the sequential suppression of the components (hyaluronic acid first and then mannan)